Electrocardiography

Paced Rhythms

Today,
there are many types of pacemakers with many different functions. In order to
interpret ECGs with paced rhythms, it is important to understand the basic functions
of pacemakers and how these functions manifest on the surface ECG. After the
basic functions are understood, pacemaker malfunctions and other subtle functions
can be interpreted on the ECG.

This chapter addresses the different types of pacemakers with their various
functions and classifications. In addition, it will review what to look for
on the ECG in order to identify pacemaker malfunctions.

Pacemaker
Classifications

Pacemakers
have one to three leads which provide pacing and sensing activity. They are
classified using three, four and sometimes even five letters which describe
the function of the pacemaker. These functions include which chambers are sensed,
which chambers are potentially paced, which chambers cause inhibition of the
pacemaker and if the pacemaker has the ability to be responsive to activity.

The first letter identifies what chamber is paced. The letter can be A (atrial
only), V (ventricular only) or D (dual or both chambers). It is not difficult
to identify what chamber is being paced when present on the ECG but, because
most pacemakers pace on demand, a device may have the ability to pace a chamber
and not be represented on the ECG.

The second letter identifies what chamber is sensed. Like the paced chamber,
A represents atrial sensing only, V represents ventricular sensing only and
D identifies a pacemaker that has the ability to sense both chambers.

The
third letter represents activity in the specific chambers that will inhibit
the pacemaker. Inhibition of a pacemaker is a simple concept and identifies
inhibition of pacing activity. Pacemakers are preprogrammed to pace at a certain
rate. Once the time period elapses that would allow the heart rate to fall below
this set rate, the pacemaker will pace. If a natural beat occurs prior to the
preprogrammed time, the pacemaker allows the heart to use its intrinsic rhythm
rather than pacing. This provides two major benefits. First, the natural activation
of the heart provides better hemodynamic function. The second benefit is that
by decreasing the amount of pacing, there is longer battery life for the pacemaker
allowing for fewer battery changes. The same letters (A,V,D) and I (inhibited)
are used for the third position.

The fourth letter that is ability of the pacemaker to be rate responsive. There
are many ways pacemakers are able to identify increasing activity. The concept
is that with increasing activity and increasing oxygen demand by the body, the
pacemaker will increase the rate at which it fires. This function is identified
in the fourth slot as an R (rate responsive). Although this function is relatively
difficult to appreciate on standard ECG due to the short time frame of the tracing
(6 seconds), it is an important concept when evaluating serial ECGs.

The table shows the common pacemaker classifications in an organized fashion
that makes the lettering system easier to understand.

First
Letter Chamber Paced

Second
Letter Chamber Paced

Third
Letter Chamber Inhibited

Description

A

A

I

Atrial
demand pacing, inhibited by sensed atrial activity

V

V

I

Ventricular
demand pacing, inhibited by sensed ventricular activity

D

D

D

Atrial
and ventricular demand pacing, inhibited by both chambers, will pace ventricle
in response to atrial activity

D

D

I

Atrial
and ventricular demand pacing, inhibited by both chambers, will not pace
ventricle in response to atrial activity

Interpreting
paced rhythms can range from very simple to very complex. The biggest obstacle
to interpreting pace rhythms is identifying the paced beats. Modern pacemakers
provide low voltage spikes on the ECG that can easily go unnoticed. Unfortunately,
there is no single lead that will consistently demonstrate the pacemaker spikes.
When any wide complex rhythm is present it is always a good idea to take a quick
look at all leads to look for occult pacemaker spikes.

Once pacemaker activity is identified, the next step is to determine if the
activity is a normal function or a malfunction. Most pacemakers are set to pace
between 50 and 70 beats per minute (there are clinical scenarios where the set
rate may be outside of this range). This chapter will review the two most common
types of pacemakers implanted: the DDD and VVI pacemakers.

As outlined above, the DDD pacemaker senses both the atrium and ventricle, paces
both atrium and ventricle and is inhibited by both the atrium and ventricle.
Starting with the atrial lead, it is designed to pace the atrium if there is
no activity after a set time that corresponds to the preset rate. If there is
atrial activity, the pacing function will be inhibited and the pacemaker will
not fire. Ventricular pacing is slightly more complex and involves an AV delay.
The AV delay is a mechanism designed to decrease the battery usage of the pacemaker
and allows for natural conduction from the atrium to the ventricle through the
AV node. Once there is atrial activity, either intrinsic or paced, the ventricular
lead is prepared to pace. If, after a preset time for AV delay, the ventricle
is not activiated, the ventricular lead will pace. The AV delay is usually set
around 200 milliseconds (the accepted cutoff for first degree AV block). Similar
to the atrial lead, if ventricular activity occurs before the predetermined
rate, the ventricular lead will be inhibited and not pace. This may occur with
either an intrinsically conducted atrial beat or with a premature ventricular
complex.

There
are many functions of pacemakers that are well beyond the scope of this module.
Dysfunctions involving sensing should be identifiable. This normal pacemaker
function can be under active or over active and both are clinically relevant.

Sensing informs the pacemaker when to pace (no activity sensed) and when not
to pace (activity sensed). If native activity is present and the pacemaker does
not identify it, inappropriate pacing will occur. This can be identified on
the surface ECG by inappropriate pacemaker spikes. There may be native atrial
or ventricular activity with pacemaker spikes that do not correlate to activity
in either chamber. The biggest concern with undersensing is the occurrence of
a pacemaker spike on a T wave leading to ventricular tachycardia. Undersensing
may be intermittent or constant.

Oversensing can be detrimental as well. The pacemaker may interpret activity
outside of the designated chamber and inhibit the pacemaker. Common sources
include chest wall activity and ventricular activity. If the pacemaker is oversensing
ventricular activity, there may be no output from the pacemaker. This makes
it necessary for an escape focus to take over pacing activity. There may be
profound bradycardia or even ventricular standstill.

The
pacemaker also may oversense ventricular activity as atrial activity and can
lead to pacemaker mediated tachycardia (PMT). In PMT, a reentry circuit is set
up and the pacemaker completes the circuit. The mechanism involves a paced ventricular
beat where the atrial lead senses the T wave and interprets it as a P wave.
Sensing the "P wave" causes the ventricular lead to fire and another
paced ventricular beat. This cycle then continues and is frequently at a rate
very close to the upper rate limit for the pacemaker. Note that a dual chamber
pacemaker is necessary for PMT.

Failure to capture is another malfunction. The pacemaker may fire but fails
to capture the chamber and result in chamber activity. This can be seen on the
surface ECG as a pacemaker spike that occurs in a portion of the ECG cycle where
atrial or ventricular capture and activity should ensue. If the pacemaker spike
occurs in a portion of the ECG where the ventricle is still refractory, it cannot
be determined if there is failure to capture because no electrical stimulus
during the refractory period will lead to activity.